Wafer processing method

ABSTRACT

A wafer processing method includes a polyester sheet providing step of positioning a wafer in an inside opening of a ring frame and providing a polyester sheet on a back side of the wafer and on a back side of the ring frame, a uniting step of heating the polyester sheet as applying a pressure to the polyester sheet to thereby unite the wafer and the ring frame through the polyester sheet by thermocompression bonding, a dividing step of cutting the wafer by using a cutting apparatus to thereby divide the wafer into individual device chips, and a pickup step of heating the polyester sheet, pushing up each device chip through the polyester sheet, and then picking up each device chip from the polyester sheet.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wafer processing method for dividinga wafer along a plurality of division lines to obtain a plurality ofindividual device chips, the division lines being formed on the frontside of the wafer to thereby define a plurality of separate regionswhere a plurality of devices are individually formed.

Description of the Related Art

In a fabrication process for device chips to be used in electronicequipment such as mobile phones and personal computers, a plurality ofcrossing division lines (streets) are first set on the front side of awafer formed of a semiconductor, for example, thereby defining aplurality of separate regions on the front side of the wafer. In eachseparate region, a device such as an integrated circuit (IC), alarge-scale integration (LSI), and a light emitting diode (LED) is nextformed. Thereafter, a ring frame having an inside opening is prepared,in which an adhesive tape called a dicing tape is previously attached inits peripheral portion to the ring frame (the back side of the ringframe) so as to close the inside opening of the ring frame. Thereafter,a central portion of the adhesive tape is attached to the back side ofthe wafer such that the wafer is accommodated in the inside opening ofthe ring frame. In this manner, the wafer, the adhesive tape, and thering frame are united together to form a frame unit. Thereafter, thewafer included in this frame unit is processed to be divided along eachdivision line, thereby obtaining a plurality of individual device chipsincluding the respective devices.

For example, a cutting apparatus is used to divide the wafer. Thecutting apparatus includes a chuck table for holding the wafer throughthe adhesive tape and a cutting unit for cutting the wafer. The cuttingunit includes a cutting blade for cutting the wafer and a spindle forrotating the cutting blade. The cutting blade has a central throughhole, and the spindle is fitted in this central through hole of thecutting blade, so that the cutting blade and the spindle are rotated asa unit. An annular abrasive portion is provided around the outercircumference of the cutting blade, so as to cut the wafer. In cuttingthe wafer by using this cutting apparatus, the frame unit is placed onthe chuck table, and the wafer is held through the adhesive tape on theupper surface of the chuck table. In this condition, the spindle isrotated to thereby rotate the cutting blade, and the cutting unit isnext lowered to a predetermined height. Thereafter, the chuck table andthe cutting unit are relatively moved in a direction parallel to theupper surface of the chuck table. Accordingly, the wafer is cut alongeach division line by the cutting blade being rotated, so that the waferis divided.

Thereafter, the frame unit is transferred from the cutting apparatus toanother apparatus for applying ultraviolet light to the adhesive tape tothereby reduce the adhesion of the adhesive tape. Thereafter, eachdevice chip is picked up from the adhesive tape. As a processingapparatus capable of producing the device chips with high efficiency,there is a cutting apparatus capable of continuously performing theoperation for dividing the wafer and the operation for applyingultraviolet light to the adhesive tape (see Japanese Patent No. 3076179,for example). Each device chip picked up from the adhesive tape is nextmounted on a predetermined wiring substrate or the like.

SUMMARY OF THE INVENTION

The adhesive tape includes a base layer and an adhesive layer formed onthe base layer. In cutting the wafer by using the cutting apparatus, thecutting unit is positioned at a predetermined height such that the lowerend of the cutting blade reaches a position lower than the lower surface(the back side) of the wafer, so as to reliably divide the wafer.Accordingly, the adhesive layer of the adhesive tape attached to theback side of the wafer is also cut by the cutting blade at the time thewafer is cut by the cutting blade. As a result, in cutting the wafer,cutting dust due to the wafer is generated, and cutting dust due to theadhesive layer is also generated. In cutting the wafer, a cutting wateris supplied to the wafer and the cutting blade. The cutting dustgenerated in cutting the wafer is taken into the cutting water and thendiffused on the front side of the wafer. However, the cutting dust dueto the adhesive layer is apt to adhere again to the front side of eachdevice. Furthermore, it is not easy to remove this cutting dust adheredto each device in a cleaning step to be performed after cutting thewafer. Accordingly, when the cutting dust due to the adhesive layeradheres to each device formed on the front side of the wafer, therearises a problem such that each device chip may be degraded in quality.

The present invention has been made in view of the above problem, and itis therefore an object of the present invention to provide a waferprocessing method which can prevent the adherence of cutting dust to thefront side of each device in cutting the wafer, thereby suppressing adegradation in quality of each device chip.

In accordance with an aspect of the present invention, there is provideda wafer processing method for dividing a wafer along a plurality ofdivision lines to obtain a plurality of individual device chips, thedivision lines being formed on a front side of the wafer to therebydefine a plurality of separate regions where a plurality of devices areindividually formed. The wafer processing method includes a ring framepreparing step of preparing a ring frame having an inside opening foraccommodating the wafer; a polyester sheet providing step of positioningthe wafer in the inside opening of the ring frame and providing apolyester sheet on a back side of the wafer and on a back side of thering frame; a uniting step of heating the polyester sheet as applying apressure to the polyester sheet after performing the polyester sheetproviding step, thereby uniting the wafer and the ring frame through thepolyester sheet by thermocompression bonding to form a frame unit in acondition where the front side of the wafer and the front side of thering frame are exposed; a dividing step of cutting the wafer along eachdivision line by using a cutting apparatus including a rotatable cuttingblade after performing the uniting step, thereby dividing the wafer intothe individual device chips; and a pickup step of heating the polyestersheet in each of the plurality of separate regions corresponding to eachdevice chip, pushing up each device chip through the polyester sheet,and then picking up each device chip from the polyester sheet afterperforming the dividing step.

Preferably, the uniting step includes a step of applying infrared lightto the polyester sheet, thereby performing the thermocompressionbonding.

Preferably, the polyester sheet is larger in size than the ring frame,and the uniting step includes an additional step of cutting thepolyester sheet after heating the polyester sheet, thereby removing apart of the polyester sheet outside the outer circumference of the ringframe.

Preferably, the pickup step includes a step of expanding the polyestersheet to thereby increase a spacing between any adjacent ones of thedevice chips.

Preferably, the polyester sheet is formed of a material selected fromthe group consisting of polyethylene terephthalate and polyethylenenaphthalate.

In the case that the polyester sheet is formed of polyethyleneterephthalate, the polyester sheet is preferably heated in the range of250° C. to 270° C. in the uniting step. In the case that the polyestersheet is formed of polyethylene naphthalate, the polyester sheet ispreferably heated in the range of 160° C. to 180° C. in the unitingstep.

Preferably, the wafer is formed of a material selected from the groupconsisting of silicon, gallium nitride, gallium arsenide, and glass.

In the wafer processing method according to a preferred embodiment ofthe present invention, the wafer and the ring frame are united by usingthe polyester sheet having no adhesive layer in place of an adhesivetape having an adhesive layer, thereby forming the frame unit composedof the wafer, the ring frame, and the polyester sheet united together.The uniting step of uniting the wafer and the ring frame through thepolyester sheet is realized by thermocompression bonding. Afterperforming the uniting step, the wafer is cut to be divided into theindividual device chips by using the cutting blade. Thereafter, in eachof the plurality of separate regions corresponding to each device chipin the polyester sheet, the polyester sheet is heated, and each devicechip is pushed up through the polyester sheet, whereby each device chipis picked up from the polyester sheet. Each device chip picked up isnext mounted on a predetermined wiring substrate or the like. Note that,when the polyester sheet is heated in picking up each device chip, thepolyester sheet can be easily peeled off from the device chip, so thatit is possible to reduce a load applied to each device chip.

In cutting the wafer by using the cutting blade, the polyester sheetprovided below the wafer is also cut by the cutting blade. That is, thewafer and the polyester sheet bonded to the back side of the wafer arecut together by the cutting blade in a condition where the front side ofthe wafer is oriented upward. Accordingly, cutting dust due to thepolyester sheet is generated. This cutting dust is taken into cuttingwater supplied in cutting the wafer and then diffused on the front sideof the wafer. However, since the polyester sheet has no adhesive layer,the cutting dust is not attached to the wafer, so that the cutting dustcan be removed more reliably in a cleaning step to be performed later.In this manner, according to the preferred embodiment of the presentinvention, the frame unit can be formed by thermocompression bondingusing the polyester sheet having no adhesive layer. Accordingly, cuttingdust due to an adhesive layer is not generated in cutting the wafer, sothat it is possible to suppress a degradation in quality of each devicechip due to such cutting dust having adhesion.

Thus, the wafer processing method according to the preferred embodimentof the present invention can exhibit the effect that cutting dust doesnot adhere to the front side of each device in cutting the wafer and adegradation in quality of each device chip divided from the wafer can besuppressed.

The above and other objects, features, and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a wafer;

FIG. 2 is a schematic perspective view depicting a manner of positioningthe wafer and a ring frame on a holding surface of a chuck table;

FIG. 3 is a schematic perspective view depicting a polyester sheetproviding step;

FIG. 4 is a schematic perspective view depicting a uniting step;

FIG. 5 is a schematic perspective view depicting a modification of theuniting step;

FIG. 6 is a schematic perspective view depicting another modification ofthe uniting step;

FIG. 7A is a schematic perspective view depicting a manner of cuttingthe polyester sheet after performing the uniting step;

FIG. 7B is a schematic perspective view of a frame unit formed byperforming the step depicted in FIG. 7A;

FIG. 8 is a schematic perspective view depicting a dividing step;

FIG. 9 is a schematic perspective view depicting a manner of loading theframe unit to a pickup apparatus after performing the dividing step;

FIG. 10A is a schematic sectional view depicting a standby conditionwhere the frame unit is fixed to a frame support table set at an initialposition in a pickup step using the pickup apparatus depicted in FIG. 9;and

FIG. 10B is a schematic sectional view depicting a working conditionwhere the frame support table holding the frame unit with the polyestersheet is lowered to expand the polyester sheet in the pickup step.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be describedwith reference to the attached drawings. There will first be described awafer to be processed by a processing method according to this preferredembodiment. FIG. 1 is a schematic perspective view of a wafer 1. Thewafer 1 is a substantially disc-shaped substrate formed of a materialsuch as silicon (Si), silicon carbide (SiC), gallium nitride (GaN), andgallium arsenide (GaAs). The wafer 1 may be formed of any othersemiconductor materials. Further, the wafer 1 may be formed of amaterial such as sapphire, glass, and quartz. The wafer 1 has a frontside 1 a and a back side 1 b. A plurality of crossing division lines 3are formed on the front side 1 a of the wafer 1 to thereby define aplurality of separate regions where a plurality of devices 5 such as ICsand LEDs are respectively formed. The crossing division lines 3 arecomposed of a plurality of parallel division lines 3 extending in afirst direction and a plurality of parallel division lines 3 extendingin a second direction perpendicular to the first direction. In theprocessing method for the wafer 1 according to this preferredembodiment, the wafer 1 is cut along the crossing division lines 3 andthereby divided into a plurality of individual device chips individuallyincluding the plural devices 5.

The wafer 1 is cut by using a cutting apparatus. Prior to loading thewafer 1 into the cutting apparatus, the wafer 1 is united with apolyester sheet 9 (see FIG. 3) and a ring frame 7 (see FIG. 3) tothereby form a frame unit. Thus, the wafer 1 is loaded in the form ofsuch a frame unit into the cutting apparatus and then cut into theindividual device chips in the cutting apparatus, in which each devicechip is supported to the polyester sheet 9. Thereafter, the polyestersheet 9 is expanded to thereby increase the spacing between any adjacentones of the device chips. Thereafter, each device chip is picked up byusing a pickup apparatus. The ring frame 7 is formed of a rigid materialsuch as metal, and it has a circular inside opening 7 a having adiameter larger than that of the wafer 1. The outside shape of the ringframe 7 is substantially circular. The ring frame 7 has a front side 7 band a back side 7 c. In forming the frame unit, the wafer 1 isaccommodated in the inside opening 7 a of the ring frame 7 andpositioned in such a manner that the center of the wafer 1 substantiallycoincides with the center of the inside opening 7 a.

The polyester sheet 9 is a flexible (expandable) resin sheet, and it hasa flat front side and a flat back side. The polyester sheet 9 is acircular sheet having a diameter larger than the outer diameter of thering frame 7. The polyester sheet 9 has no adhesive layer. The polyestersheet 9 is a sheet of a polymer (polyester) synthesized by polymerizingdicarboxylic acid (compound having two carboxyl groups) and diol(compound having two hydroxy groups) as a monomer. Examples of thepolyester sheet 9 include a polyethylene terephthalate sheet andpolyethylene naphthalate sheet. The polyester sheet 9 is transparent ortranslucent to visible light. As a modification, the polyester sheet 9may be opaque. Since the polyester sheet 9 has no adhesive property, itcannot be attached to the wafer 1 and the ring frame 7 at roomtemperature. However, the polyester sheet 9 is a thermoplastic sheet, sothat, when the polyester sheet 9 is heated to a temperature near itsmelting point under a predetermined pressure in a condition where thepolyester sheet 9 is in contact with the wafer 1 and the ring frame 7,the polyester sheet 9 is partially melted and thereby bonded to thewafer 1 and the ring frame 7. That is, by applying heat and pressure tothe polyester sheet 9 in the condition where the polyester sheet 9 is incontact with the wafer 1 and the ring frame 7, the polyester sheet 9 canbe bonded to the wafer 1 and the ring frame 7. Thusly, in the processingmethod for the wafer 1 according to this preferred embodiment, all ofthe wafer 1, the ring frame 7, and the polyester sheet 9 are united bythermocompression bonding as mentioned above, thereby forming the frameunit.

The steps of the processing method for the wafer 1 according to thispreferred embodiment will now be described. Prior to uniting the wafer1, the polyester sheet 9, and the ring frame 7, a polyester sheetproviding step is performed by using a chuck table 2 having a holdingsurface 2 a depicted in FIG. 2. FIG. 2 is a schematic perspective viewdepicting a manner of positioning the wafer 1 and the ring frame 7 onthe holding surface 2 a of the chuck table 2. That is, the polyestersheet providing step is performed on the holding surface 2 a of thechuck table 2 as depicted in FIG. 2. The chuck table 2 has a circularporous member having a diameter larger than the outer diameter of thering frame 7. The porous member constitutes a central upper portion ofthe chuck table 2. The porous member has an upper surface functioning asthe holding surface 2 a of the chuck table 2. A suction passage (notdepicted) is formed in the chuck table 2, in which one end of thesuction passage is connected to the porous member. Further, a vacuumsource 2 b (see FIG. 3) is connected to the other end of the suctionpassage. The suction passage is provided with a selector 2 c (see FIG.3) for switching between an ON condition and an OFF condition. When theON condition is established by the selector 2 c, a vacuum produced bythe vacuum source 2 b is applied to a workpiece placed on the holdingsurface 2 a of the chuck table 2, thereby holding the workpiece on thechuck table 2 under suction.

In the polyester sheet providing step, the wafer 1 and the ring frame 7are first placed on the holding surface 2 a of the chuck table 2 asdepicted in FIG. 2. At this time, the front side 1 a of the wafer 1 isoriented downward, and the front side 7 b of the ring frame 7 is alsooriented downward. In this condition, the wafer 1 is positioned in theinside opening 7 a of the ring frame 7. Thereafter, as depicted in FIG.3, the polyester sheet 9 is provided on the back side 1 b (uppersurface) of the wafer 1 and on the back side 7 c (upper surface) of thering frame 7. FIG. 3 is a schematic perspective view depicting a mannerof providing the polyester sheet 9 on the wafer 1 and the ring frame 7.That is, as depicted in FIG. 3, the polyester sheet 9 is provided so asto fully cover the wafer 1 and the ring frame 7. In the polyester sheetproviding step, the diameter of the polyester sheet 9 is set larger thanthe diameter of the holding surface 2 a of the chuck table 2. Unless thediameter of the polyester sheet 9 is larger than the diameter of theholding surface 2 a, there may arise a problem such that, when thevacuum from the vacuum source 2 b is applied to the holding surface 2 aof the chuck table 2 in a uniting step to be performed later, the vacuummay leak from any gap between the polyester sheet 9 and the holdingsurface 2 a because the holding surface 2 a is not fully covered withthe polyester sheet 9, so that a pressure cannot be properly applied tothe polyester sheet 9.

In the processing method for the wafer 1 according to this preferredembodiment, a uniting step is next performed in such a manner that thepolyester sheet 9 is heated to unite the wafer 1 and the ring frame 7through the polyester sheet 9 by thermocompression bonding. FIG. 4 is aschematic perspective view depicting the uniting step according to thispreferred embodiment. As depicted in FIG. 4, the polyester sheet 9transparent or translucent to visible light is provided so as to fullycover the wafer 1, the ring frame 7, and the holding surface 2 a of thechuck table 2, which are all depicted by broken lines in FIG. 4. In theuniting step, the selector 2 c is operated to establish the ON conditionwhere the vacuum source 2 b is in communication with the porous memberof the chuck table 2, i.e., the holding surface 2 a of the chuck table2, so that a vacuum produced by the vacuum source 2 b is applied to thepolyester sheet 9 provided on the chuck table 2. Accordingly, thepolyester sheet 9 is brought into close contact with the wafer 1 and thering frame 7 by the atmospheric pressure applied to the upper surface ofthe polyester sheet 9.

Thereafter, the polyester sheet 9 is heated in a condition where thepolyester sheet 9 is sucked by the vacuum source 2 b, thereby performingthermocompression bonding. In this preferred embodiment depicted in FIG.4, for example, the heating of the polyester sheet 9 is effected by aheat gun 4 provided above the chuck table 2. The heat gun 4 includesheating means such as a heating wire and an air blowing mechanism suchas a fan. Accordingly, the heat gun 4 can heat ambient air and blow theheated air. In a condition where the vacuum from the vacuum source 2 bis applied to the polyester sheet 9, the heat gun 4 is operated tosupply hot air 4 a to the upper surface of the polyester sheet 9.Accordingly, when the polyester sheet 9 is heated to a predeterminedtemperature, the polyester sheet 9 is bonded to the wafer 1 and the ringframe 7 by thermocompression bonding.

Another method for heating the polyester sheet 9 may be adopted. Forexample, any member heated to a predetermined temperature may be pressedon the polyester sheet 9 against the wafer 1 and the ring frame 7. FIG.5 is a schematic perspective view depicting such a modification of theuniting step. As depicted in FIG. 5, the polyester sheet 9 transparentor translucent to visible light is provided so as to fully cover thewafer 1, the ring frame 7, and the holding surface 2 a of the chucktable 2, which are all depicted by broken lines in FIG. 5. In thismodification depicted in FIG. 5, a heat roller 6 including a heat sourceis used. More specifically, the vacuum produced by the vacuum source 2 bis first applied to the polyester sheet 9, so that the polyester sheet 9is brought into close contact with the wafer 1 and the ring frame 7 bythe atmospheric pressure applied to the upper surface of the polyestersheet 9.

Thereafter, the heat roller 6 is heated to a predetermined temperature,and next placed on the holding surface 2 a of the chuck table 2 at oneend lying on the outer circumference of the holding surface 2 a asdepicted in FIG. 5. Thereafter, the heat roller 6 is rotated about itsaxis to roll on the chuck table 2 through the polyester sheet 9 from theabove one end to another end diametrically opposite to the above oneend. As a result, the polyester sheet 9 is bonded to the wafer 1 and thering frame 7 by thermocompression bonding. In the case that a force forpressing the polyester sheet 9 is applied by the heat roller 6, thethermocompression bonding is effected at a pressure higher thanatmospheric pressure. Preferably, a cylindrical surface of the heatroller 6 is coated with fluororesin. Further, the heat roller 6 may bereplaced by any iron-like pressure member having a flat base plate andcontaining a heat source. In this case, the pressure member is heated toa predetermined temperature to thereby provide a hot plate, which isnext pressed on the polyester sheet 9 held on the chuck table 2.

Still another method for heating the polyester sheet 9 may be adopted inthe following manner. FIG. 6 is a schematic perspective view depictingsuch another modification of the uniting step. As depicted in FIG. 6,the polyester sheet 9 transparent or translucent to visible light isprovided so as to fully cover the wafer 1, the ring frame 7, and theholding surface 2 a of the chuck table 2, which are all depicted bybroken lines in FIG. 6. In this modification depicted in FIG. 6, aninfrared lamp 8 is provided above the chuck table 2 to heat thepolyester sheet 9. The infrared lamp 8 can apply infrared light 8 ahaving an absorption wavelength to at least the material of thepolyester sheet 9. Also in the modification depicted in FIG. 6, thevacuum produced by the vacuum source 2 b is first applied to thepolyester sheet 9, so that the polyester sheet 9 is brought into closecontact with the wafer 1 and the ring frame 7 by the atmosphericpressure applied to the upper surface of the polyester sheet 9.Thereafter, the infrared lamp 8 is operated to apply the infrared light8 a to the polyester sheet 9, thereby heating the polyester sheet 9. Asa result, the polyester sheet 9 is bonded to the wafer 1 and the ringframe 7 by thermocompression bonding.

When the polyester sheet 9 is heated to a temperature near its meltingpoint by performing any one of the above methods, the polyester sheet 9is bonded to the wafer 1 and the ring frame 7 by thermocompressionbonding. After bonding the polyester sheet 9, the selector 2 c isoperated to disconnect the communication of the porous member of thechuck table 2 from the vacuum source 2 b. Accordingly, the suctionholding by the chuck table 2 is canceled.

Thereafter, the polyester sheet 9 is circularly cut along the outercircumference of the ring frame 7 to remove an unwanted peripheralportion of the polyester sheet 9. FIG. 7A is a schematic perspectiveview depicting a manner of cutting the polyester sheet 9. As depicted inFIG. 7A, a disc-shaped (annular) cutter 10 is used to cut the polyestersheet 9. The cutter 10 has a central through hole 10 a in which arotating shaft 10 b is fitted. Accordingly, the cutter 10 is rotatableabout the axis of the rotating shaft 10 b. First, the cutter 10 ispositioned above the ring frame 7. At this time, the rotating shaft 10 bis set so as to extend in the radial direction of the chuck table 2.Thereafter, the cutter 10 is lowered until the outer circumference(cutting edge) of the cutter 10 comes into contact with the polyestersheet 9 placed on the ring frame 7. That is, the polyester sheet 9 iscaught between the cutter 10 and the ring frame 7, so that the polyestersheet 9 is cut by the cutter 10 to form a cut mark 9 a. Further, thecutter 10 is rolled on the polyester sheet 9 along a circular line setbetween the inner circumference of the ring frame 7 (i.e., the peripheryof the inside opening 7 a of the ring frame 7) and the outercircumference of the ring frame 7, thereby circularly forming the cutmark 9 a along the above circular line. As a result, a predeterminedcentral portion of the polyester sheet 9 is surrounded by the circularcut mark 9 a. Thereafter, a remaining peripheral portion of thepolyester sheet 9 outside the circular cut mark 9 a is removed. That is,an unwanted peripheral portion of the polyester sheet 9 including anoutermost peripheral portion outside the outer circumference of the ringframe 7 can be removed.

The cutter 10 may be replaced by an ultrasonic cutter for cutting thepolyester sheet 9. Further, a vibration source for vibrating the cutter10 at a frequency in an ultrasonic band may be connected to the cutter10. Further, in cutting the polyester sheet 9, the polyester sheet 9 maybe cooled to be hardened in order to facilitate the cutting operation.By cutting the polyester sheet 9 as mentioned above, a frame unit 11depicted in FIG. 7B is formed, in which the frame unit 11 is composed ofthe wafer 1, the ring frame 7, and the polyester sheet 9 unitedtogether. That is, the wafer 1 and the ring frame 7 are united with eachother through the polyester sheet 9 to form the frame unit 11 asdepicted in FIG. 7B. FIG. 7B is a schematic perspective view of theframe unit 11 in a condition where the front side 1 a of the wafer 1 andthe front side 7 b of the ring frame 7 are exposed upward.

In performing the thermocompression bonding as mentioned above, thepolyester sheet 9 is heated preferably to a temperature lower than orequal to the melting point of the polyester sheet 9. If the heatingtemperature is higher than the melting point of the polyester sheet 9,there is a possibility that the polyester sheet 9 may be melted to suchan extent that the shape of the polyester sheet 9 cannot be maintained.Further, the polyester sheet 9 is heated preferably to a temperaturehigher than or equal to the softening point of the polyester sheet 9. Ifthe heating temperature is lower than the softening point of thepolyester sheet 9, the thermocompression bonding cannot be properlyperformed. Accordingly, the polyester sheet 9 is heated preferably to atemperature higher than or equal to the softening point of the polyestersheet 9 and lower than or equal to the melting point of the polyestersheet 9. Further, there is a case that the softening point of thepolyester sheet 9 may be unclear. To cope with such a case, inperforming the thermocompression bonding, the polyester sheet 9 isheated preferably to a temperature higher than or equal to a presettemperature and lower than or equal to the melting point of thepolyester sheet 9, the preset temperature being lower by 20° C. than themelting point of the polyester sheet 9.

In the case that the polyester sheet 9 is a polyethylene terephthalatesheet, for example, the heating temperature in the uniting step ispreferably set in the range of 250° C. to 270° C. Further, in the casethat the polyester sheet 9 is a polyethylene naphthalate sheet, theheating temperature in the uniting step is preferably set in the rangeof 160° C. to 180° C.

The heating temperature is defined herein as the temperature of thepolyester sheet 9 to be heated in performing the uniting step. As theheat sources included in the heat gun 4, the heat roller 6, and theinfrared lamp 8 mentioned above, some kind of heat source capable ofsetting an output temperature has been put into practical use. However,even when such a heat source is used to heat the polyester sheet 9, thetemperature of the polyester sheet 9 does not reach the outputtemperature set above in some case. To cope with such a case, the outputtemperature of the heat source may be set to a temperature higher thanthe melting point of the polyester sheet 9 in order to heat thepolyester sheet 9 to a predetermined temperature.

After performing the uniting step mentioned above, a dividing step isperformed in such a manner that the wafer 1 in the condition of theframe unit 11 is cut by a cutting blade to obtain individual devicechips. The dividing step is performed by using a cutting apparatus 12depicted in FIG. 8 in this preferred embodiment. FIG. 8 is a schematicperspective view depicting the dividing step. As depicted in FIG. 8, thecutting apparatus 12 includes a cutting unit 14 for cutting a workpieceand a chuck table (not depicted) for holding the workpiece. The cuttingunit 14 includes a cutting blade 18 having an annular abrasive portion(cutting edge) for cutting the workpiece and a spindle (not depicted)for supporting the cutting blade 18 so as to rotate the cutting blade18. The cutting blade 18 has a central through hole for mounting thefront end of the spindle. The cutting blade 18 is composed of an annularbase (hub) having the above-mentioned central through hole and anannular abrasive portion provided along the outer circumference of theannular base. The spindle is rotatably supported in a spindle housing16, and the base end of the spindle is connected to a spindle motor (notdepicted) accommodated in the spindle housing 16. Accordingly, thecutting blade 18 can be rotated by operating the spindle motor. Thechuck table has an upper surface as a holding surface for holding thewafer 1.

When the workpiece is cut by the cutting blade 18, heat is generated bythe friction between the cutting blade 18 and the workpiece. Further,when the workpiece is cut by the cutting blade 18, cutting dust isgenerated from the workpiece. To remove such heat and cutting dust dueto the cutting of the workpiece, cutting water such as pure water issupplied to the cutting blade 18 and the workpiece during the cutting ofthe workpiece. Accordingly, the cutting unit 14 includes a pair ofcutting water nozzles 20 for supplying cutting water to the cuttingblade 18 and the workpiece, in which the pair of cutting water nozzles20 are located so as to face both sides of the cutting blade 18. In FIG.8, only one of the two cutting water nozzles 20 is depicted.

In cutting the wafer 1, the frame unit 11 is placed on the chuck tablein the condition where the front side 1 a of the wafer 1 is exposedupward. Accordingly, the wafer 1 is held through the polyester sheet 9on the chuck table. Thereafter, the chuck table is rotated to make thedivision lines 3 extending in the first direction on the front side 1 aof the wafer 1 parallel to a feeding direction in the cutting apparatus12. Further, the chuck table and the cutting unit 14 are relativelymoved in a direction perpendicular to the feeding direction in ahorizontal plane to thereby position the cutting blade 18 directly abovean extension of a predetermined one of the division lines 3 extending inthe first direction.

Thereafter, the spindle is rotated to thereby rotate the cutting blade18. Thereafter, the cutting unit 14 is lowered to a predeterminedheight, and the chuck table and the cutting unit 14 are relatively movedin the feeding direction parallel to the upper surface of the chucktable. Accordingly, the abrasive portion of the cutting blade 18 beingrotated comes into contact with the wafer 1 to thereby cut the wafer 1along the predetermined division line 3 in the feeding direction. As aresult, a cut mark 3 a (groove) is formed along the predetermineddivision line 3 so as to fully cut the wafer 1 and the polyester sheet9. After cutting the wafer 1 and the polyester sheet 9 along thepredetermined division line 3, the chuck table and the cutting unit 14are relatively moved in an indexing direction perpendicular to thefeeding direction by the pitch of the division lines 3. Thereafter, theabove cutting operation is similarly performed along the next divisionline 3 adjacent to the above predetermined division line 3. Aftersimilarly performing the cutting operation along all of the otherdivision lines 3 extending in the first direction, the chuck table isrotated 90 degrees about its axis perpendicular to the holding surface,so that the other division lines 3 extending in the second directionperpendicular to the first direction become parallel to the feedingdirection. Thereafter, the above cutting operation is similarlyperformed along all the other division lines 3 extending in the seconddirection. After performing the cutting operation along all the otherdivision lines 3 extending in the second direction, the dividing step isfinished.

The cutting apparatus 12 may include a cleaning unit (not depicted)provided in the vicinity of the cutting unit 14. That is, the wafer 1cut by the cutting unit 14 may be transferred to the cleaning unit andthen may be cleaned by the cleaning unit. For example, the cleaning unitincludes a cleaning table having a holding surface for holding the frameunit 11 and a cleaning water nozzle adapted to be horizontally moved inopposite directions above the frame unit 11 held on the holding surfaceof the cleaning table. The cleaning water nozzle functions to supplycleaning water such as pure water to the wafer 1. The cleaning table isrotatable about its axis perpendicular to the holding surface. Inoperation, the cleaning table is rotated about its axis and at the sametime the cleaning water is supplied from the cleaning water nozzle tothe wafer 1. During this supply of the cleaning water, the cleaningwater nozzle is horizontally moved in opposite directions along a pathpassing through the position directly above the center of the holdingsurface of the cleaning table. Accordingly, the entire surface of thefront side 1 a of the wafer 1 can be cleaned by the cleaning water.

By performing the dividing step as mentioned above, the wafer 1 isdivided into the individual device chips, which are still supported tothe polyester sheet 9. In cutting the wafer 1, the cutting unit 14 ispositioned at a predetermined height such that the lower end of thecutting blade 18 is lower in level than the back side 1 b of the wafer1, in order to reliably divide the wafer 1. Accordingly, when the wafer1 is cut by the cutting blade 18, the polyester sheet 9 bonded to theback side 1 b of the wafer 1 is also cut by the cutting blade 18, sothat cutting dust due to the polyester sheet 9 is generated. In the casethat an adhesive tape having an adhesive layer is used in the frame unit11 in place of the polyester sheet 9, cutting dust due to the adhesivelayer of the adhesive tape is generated. In this case, the cutting dustis taken into the cleaning water supplied from the cutting water nozzles20, and then diffused on the front side 1 a of the wafer 1. The cuttingdust due to the adhesive layer is apt to adhere again to the front sideof each device 5. Furthermore, it is not easy to remove the cutting dustadhered to the front side of each device 5 in a cleaning step ofcleaning the wafer 1 after the dividing step. When the cutting dust dueto the adhesive layer adheres to each device 5, there arises a problemsuch that each device chip divided from the wafer 1 may be degraded inquality.

In contrast, the processing method for the wafer 1 according to thispreferred embodiment has the following advantage. In this preferredembodiment, the polyester sheet 9 having no adhesive layer is used inthe frame unit 11 in place of an adhesive tape having an adhesive layer.Although the cutting dust due to the polyester sheet 9 is generated andthen diffused on the front side 1 a of the wafer 1 as being taken intothe cleaning water, this cutting dust is not attached to the wafer 1relatively, but it is reliably removed also in the subsequent cleaningstep. Accordingly, it is possible to suppress a degradation in qualityof each device chip due to the cutting dust.

After performing the dividing step or the cleaning step, a pickup stepis performed to pick up each device chip from the polyester sheet 9. Thepickup step is performed by using a pickup apparatus 22 depicted in FIG.9. FIG. 9 is a schematic perspective view depicting a manner of loadingthe frame unit 11 to the pickup apparatus 22. As depicted in FIG. 9, thepickup apparatus 22 includes a cylindrical drum 24 and a frame holdingunit 26 having a frame support table 30 provided around the cylindricaldrum 24. The cylindrical drum 24 has an inner diameter larger than thediameter of the wafer 1 and an outer diameter smaller than the innerdiameter of the ring frame 7 (the diameter of the inside opening 7 a).The frame support table 30 of the frame holding unit 26 is an annulartable having a circular inside opening larger in diameter than the drum24. That is, the frame support table 30 has an inner diameter largerthan the outer diameter of the drum 24. Further, the frame support table30 has an outer diameter larger than the outer diameter of the ringframe 7. The inner diameter of the frame support table 30 issubstantially equal to the inner diameter of the ring frame 7. The framesupport table 30 has an upper surface as a supporting surface forsupporting the ring frame 7 thereon through the polyester sheet 9.Initially, the height of the upper surface of the frame support table 30is set equal to the height of the upper end of the drum 24 (see FIG.10A). Further, the upper end portion of the drum 24 is surrounded by theinner circumference of the ring frame 7 in this initial stage.

A plurality of clamps 28 are provided on the outer circumference of theframe support table 30. Each clamp 28 functions to hold the ring frame 7supported on the frame support table 30. That is, when the ring frame 7of the frame unit 11 is placed through the polyester sheet 9 on theframe support table 30 and then held by each clamp 28, the frame unit 11can be fixed to the frame support table 30. The frame support table 30is supported by a plurality of rods 32 extending in a verticaldirection. That is, each rod 32 is connected at its upper end to thelower surface of the frame support table 30. An air cylinder 34 forvertically moving each rod 32 is connected to the lower end of each rod32. More specifically, the lower end of each rod 32 is connected to apiston (not depicted) movably accommodated in the air cylinder 34. Eachair cylinder 34 is supported to a disc-shaped base 36. That is, thelower end of each air cylinder 34 is connected to the upper surface ofthe disc-shaped base 36. Accordingly, when each air cylinder 34 isoperated in the initial stage, the frame support table 30 is loweredwith respect to the drum 24 fixed in position.

Further, a pushup mechanism 38 for pushing up each device chip supportedto the polyester sheet 9 is provided inside the drum 24. The pushupmechanism 38 has, at an upper end thereof, a heating section 38 aincluding therein a heating source such as a Peltier element or anelectric heating wire. That is, each device chip is adapted to be pushedup through the polyester sheet 9 by the pushup mechanism 38 locatedbelow the polyester sheet 9. Further, a collet 40 (see FIG. 10B) capableof holding each device chip under suction is provided above the drum 24.Both the pushup mechanism 38 and the collet 40 are movable in ahorizontal direction parallel to the upper surface of the frame supporttable 30. The collet 40 is connected through a selector 40 b (see FIG.10B) to a vacuum source 40 a (see FIG. 10B).

In the pickup step, each air cylinder 34 in the pickup apparatus 22 isfirst operated to adjust the height of the frame support table 30 suchthat the height of the upper end of the drum 24 becomes equal to theheight of the upper surface of the frame support table 30. Thereafter,the frame unit 11 transferred from the cutting apparatus 12 is placed onthe drum 24 and the frame support table 30 in the pickup apparatus 22 ina condition where the front side 1 a of the wafer 1 of the frame unit 11is oriented upward. Thereafter, each clamp 28 is operated to fix thering frame 7 of the frame unit 11 to the upper surface of the framesupport table 30. FIG. 10A is a schematic sectional view depicting astandby condition where the frame unit 11 is fixed to the frame supporttable 30 set at the initial position. At this time, the plural cut marks3 a have already been formed in the wafer 1 in the dividing step, sothat the wafer 1 has already been divided into a plurality of individualdevice chips 1 c (see FIG. 10B).

Thereafter, each air cylinder 34 is operated to lower the frame supporttable 30 of the frame holding unit 26 with respect to the drum 24. As aresult, the polyester sheet 9 fixed to the frame holding unit 26 by eachclamp 28 is expanded radially outward as depicted in FIG. 10B. FIG. 10Bis a schematic sectional view depicting a working condition where theframe support table 30 holding the ring frame 7 with the polyester sheet9 is lowered to expand the polyester sheet 9. When the polyester sheet 9is expanded radially outward as mentioned above, the spacing between anyadjacent ones of the device chips 1 c supported to the polyester sheet 9is increased as depicted in FIG. 10B. Accordingly, the contact betweenthe adjacent device chips 1 c can be suppressed, and each device chip 1c can be easily picked up. Thereafter, a target one of the device chips1 c is decided, and the pushup mechanism 38 is next moved to a positiondirectly below this target device chip 1 c as depicted in FIG. 10B.Furthermore, the collet 40 is also moved to a position directly abovethis target device chip 1 c as depicted in FIG. 10B. Thereafter, theheating section 38 a is operated to increase a temperature thereof, andthe pushup mechanism 38 causes the heating section 38 a to come intocontact with a region corresponding to the device chip 1 c in thepolyester sheet 9, thereby heating the region. Further, the pushupmechanism 38 is operated to push up the target device chip 1 c throughthe polyester sheet 9. Further, the selector 40 b is operated to makethe collet 40 communicate with the vacuum source 40 a. As a result, thetarget device chip 1 c is held under suction by the collet 40 andthereby picked up from the polyester sheet 9. Such a pickup operation issimilarly performed for all the other device chips 1 c. Thereafter, eachdevice chip 1 c picked up is mounted on a predetermined wiring substrateor the like for actual use.

Note that, when the region in the polyester sheet 9 is heated by theheating section 38 a, the region is heated to a temperature near themelting point of the polyester sheet 9, for example. Since the polyestersheet 9 reduces in its attaching force at a temperature near its meltingpoint, a load applied to the device chip in peeling off from thepolyester sheet 9 is reduced.

In the wafer processing method according to this preferred embodimentmentioned above, the frame unit 11 including the wafer 1 can be formedwithout using an adhesive tape having an adhesive layer. Accordingly, incutting the wafer 1, cutting dust due to the adhesive layer of theadhesive tape is not generated, so that this cutting dust does notadhere to each device chip 1 c. As a result, there is no possibilitythat each device chip 1 c may be degraded in quality.

The present invention is not limited to the above preferred embodiment,but various modifications may be made within the scope of the presentinvention. For example, while the polyester sheet 9 is selected from apolyethylene terephthalate sheet and a polyethylene naphthalate sheet inthe above preferred embodiment, this is merely illustrative. That is,the polyester sheet usable in the present invention may be formed of anyother materials (polyesters) such as polytrimethylene terephthalate,polybutylene terephthalate, or polybutylene naphthalate.

The present invention is not limited to the details of the abovedescribed preferred embodiment. The scope of the invention is defined bythe appended claims and all changes and modifications as fall within theequivalence of the scope of the claims are therefore to be embraced bythe invention.

What is claimed is:
 1. A wafer processing method for dividing a waferalong a plurality of division lines to obtain a plurality of individualdevice chips, the division lines being formed on the front side of thewafer to thereby define a plurality of separate regions where aplurality of devices are individually formed, the wafer processingmethod comprising: a ring frame preparing step of preparing a ring framehaving an inside opening for accommodating the wafer; a polyester sheetproviding step of positioning the wafer in the inside opening of thering frame and providing a polyester sheet on a back side of the waferand on a back side of the ring frame; a uniting step of heating thepolyester sheet as applying a pressure to the polyester sheet afterperforming the polyester sheet providing step, thereby uniting the waferand the ring frame through the polyester sheet by thermocompressionbonding to form a frame unit in a condition where the front side of thewafer and the front side of the ring frame are exposed; a dividing stepof cutting the wafer along each division line by using a cuttingapparatus including a rotatable cutting blade after performing theuniting step, thereby dividing the wafer into the individual devicechips; and a pickup step of heating the polyester sheet in each of theplurality of separate regions corresponding to each device chip, pushingup each device chip through the polyester sheet, then picking up eachdevice chip from the polyester sheet after performing the dividing step.2. The wafer processing method according to claim 1, wherein the unitingstep includes a step of applying infrared light to the polyester sheet,thereby performing the thermocompression bonding.
 3. The waferprocessing method according to claim 1, wherein the polyester sheet islarger in size than the ring frame, and the uniting step includes anadditional step of cutting the polyester sheet after heating thepolyester sheet, thereby removing a part of the polyester sheet outsidean outer circumference of the ring frame.
 4. The wafer processing methodaccording to claim 1, wherein the pickup step includes a step ofexpanding the polyester sheet to thereby increase a spacing between anyadjacent ones of the device chips.
 5. The wafer processing methodaccording to claim 1, wherein the polyester sheet is formed of amaterial selected from the group consisting of polyethyleneterephthalate and polyethylene naphthalate.
 6. The wafer processingmethod according to claim 5, wherein the polyester sheet is formed ofpolyethylene terephthalate, and the polyester sheet is heated in therange of 250° C. to 270° C. in the uniting step.
 7. The wafer processingmethod according to claim 5, wherein the polyester sheet is formed ofpolyethylene naphthalate, and the polyester sheet is heated in the rangeof 160° C. to 180° C. in the uniting step.
 8. The wafer processingmethod according to claim 1, wherein the wafer is formed of a materialselected from the group consisting of silicon, gallium nitride, galliumarsenide, and glass.